Mountain biking is a popular recreational and competitive sport that involves riders travelling over various terrain. To improve rider comfort, most modern mountain bikes are now equipped with suspension systems that aim to absorb bumps and vibrations and improve handling performance. Such suspension systems include energy storage and dissipation elements integrated onto the bicycle frame, which minimise the transmission of vertical perturbations to the rider. Examples include coil or gas springs for energy storage and liquid or gas based dampers for energy dissipation.
The suspension system components above the shock absorber (front triangle, rider mass etc) are often referred to as the sprung mass, whilst the components below the shock absorber are often referred to as the unsprung mass (wheel, tyre etc). Movement of the sprung mass towards the unsprung mass is known as compression, whilst movement of the sprung mass away from the unsprung mass is known as rebound. Damping is included in both rebound and compression in order to control rider comfort and handling, and this is usually provided in disproportionate amounts.
To those skilled in the art, it is well known that larger damping rates are desirable in rebound than in compression. In general, lower compression damping rates help minimise the transmission of force to the rider when hitting a bump, thus preventing ride harshness. In rebound, higher damping rates prevent excessive sprung/unsprung mass velocities when the stored spring energy is released. Furthermore, optimum rebound damping rates vary substantially according to rider mass, frame design, and the terrain being traversed by the rider. Using terrain as an example, whilst traversing high frequency bumps a lower rebound damping rate is desirable. On passing the crest of a bump, lower rebound damping permits the suspension to quickly restore itself in preparation for the next obstacle, and maximises wheel to ground contact. A rebound damping rate that is too high in this situation would cause the suspension to ‘pack down’ over subsequent bumps because it has insufficient time to rebound before the next obstacle. This can lead to poor handling as the wheel may skip between bumps resulting in periods of zero wheel contact force. Furthermore, passenger discomfort is degraded due to the more compressed state of the spring, which transmits larger forces to the rider. On the other hand, whilst traversing lower frequency bumps, a higher rebound damping rate is desirable in order prevent excessive vertical motions. As a consequence, many bicycle rear shock absorbers incorporate the facility for manual rebound damping adjustment.
However, when a bike fitted with a suspension system is ridden on a smooth terrain, for example on a road, the suspension system will dissipate some energy input by the rider pedalling. For example, pedalling motion causes the suspension system to compress and extend as a function of the pedalling frequency. This phenomenon is known as “pedal bobbing”. The energy used in this way does not contribute to the forward propulsion of the bicycle and so reduces the efficiency of each rotation of the pedals. Therefore, the benefits gained from using a conventional suspension system on rough terrain come at the expense of reduced rider efficiency when travelling on smooth terrain.
Pedal bob is a known problem in the art and has prompted manufacturers to put forward a wide range of solutions to counter it. Some solutions revolve around advanced frame geometries whilst others focus specifically on the design of shock absorbers.
For example, U.S. Pat. No. 6,978,872 describes a suspension damper comprising a stable platform valve (SPV) assembly that only allows compression of the damper when the applied force exceeds a threshold value. This is effective in countering pedal bob since pedal bob is a low velocity phenomenon and is generally insufficient to exceed the required threshold. Due to the threshold requirement of the U.S. Pat. No. 6,978,872 damper, bump absorption is compromised when travelling over rougher terrain.
WO-A-95/29838 also identifies the problems associated with pedal bob on a cycle fitted with a damping system. The characteristics of the suspension system disclosed therein are dependent on whether the rider is sitting or standing out of the saddle. It is reasoned that pedal bob is more significant when the rider is riding out of the saddle since the rider's weight is placed primarily on the pedals. The distribution of the rider's weight on various mechanical components causes the responsiveness of the shock absorber to change accordingly.
The document WO-A-2004/033280 discloses an “anti-bob” system for controlling the shock absorbing devices of a cycle. A detector detects the torque applied to the crank axle (i.e. the pedal axle) or the pressure between the rider's feet and the pedals, and then produces a control signal in response. The control signal then controls the activation of a controllable shock damping device.
A shock absorber system adapted for differentiating between rider-induced forces and terrain-induced forces is disclosed in US-A-2008/0093820. The shock absorber described therein has two fluid chambers connected by a hose. An inertial valve opens in response to terrain-induced forces, providing fluid communication between the two chambers, and provides softer damping.
U.S. Pat. No. 5,996,745 provides a controllable damper system that includes a fluid filled cylinder comprising a valve having a piezoelectric bender. A sensor monitors the displacement and velocity of a piston within the cylinder and controllably applies a voltage to the bender in response. The stiffness of the bender changes in accordance with the applied voltage and affects the rate at which damping fluid flows through flow channels within the chamber thereby affecting the damping characteristics. This device is based upon the principle of controllable orifice damping which serves to increase high velocity damping and is less effective at low velocities.
Magnetorheological (MR) fluids are known. They comprise a suspension of micron-sized ferromagnetic particles distributed in a carrier liquid. When a magnetic field is applied to a MR fluid, the ferromagnetic particles align along the lines of magnetic flux forming chain like structures. The degree of alignment is proportional to the magnitude of the applied magnetic field. It is found that when a magnetic field is applied to a MR fluid, the chains of ferromagnetic particles restrict movement of the fluid transverse the chains, thereby increasing its apparent viscosity. This increase in apparent viscosity occurs due to the development of a yield stress within the fluid. Those skilled in the art will understand that this flow behaviour is similar to that of a Bingham plastic. In the absence of a magnetic field, the fluid behaves in a Newtonian manner. Electrorheological (ER) fluids under the influence of an electric field behave in a similar manner to MR fluids under the influence of a magnetic field.
WO-A-2007/009341 discloses an MR damper with at least one sensor embedded therein for monitoring an external force exerted thereon. In response, a signal is generated which controls the magnetic field and hence the resulting yield force and rheological damping of the damper.
An application of MR dampers in vehicles is disclosed WO 98/56642. In this case, the MR damper system is specifically adapted to enhance the effectiveness of the suspension on vehicle cabs. One or more sensors measure quantities including the cab displacement, speed, steering angle, level of braking, throttle position, and vertical and lateral acceleration. A controller processes information from the sensors to determine an appropriate current to be supplied to an MR filled fluid damper.
A system for controlling a damper arrangement for an engine or transmission is described in US-A-2006/0173592. This document discloses a real-time varying MR damping system which aims to provide optimal damping around the resonance frequency of the given body/engine design. Accelerometers act as vibration sensors and produce input signals which alter the magnitude of a current, thus controlling the level of damping of the MR mount.
Similarly, GB-A-2382638 discloses a vehicle MR damper comprising coaxial cylinders where the viscosity of the MR fluid is controlled in response to measured parameters.
“TRAIL-TRONIC”® produced by GERMAN ANSWER bike technology GmbH & Co. KG (http://www.german-a.de/en/) is a fully integrated electronically controlled suspension system for mountain bikes. A sensor mounted on the front wheel measures its vertical acceleration and controls the level of suspension provided by the rear suspension. The system allows the user to choose the threshold limit at which the suspension locks out.
It is an object of the present invention to provide a controllable damper system that is simple in construction and compact, allowing easy installation on the frame of a bicycle, wherein the level of damping is automatically adjusted to its optimum level depending on the type of terrain the rider is riding on. Generally it is desirable for the damping system to provide sufficient damping when travelling over rough terrain, but to adapt to efficiently minimise the effects of pedal bob when travelling over smooth terrain. ER/MR fluids are particularly suitable in the context of the present invention because they exhibit inherently suitable low velocity flow characteristics that can be utilised to resist pedal bob motions.
A further object of the present invention is to provide a damper wherein the level of rebound damping can be manually adjusted by the user leaving the level of compression damping substantially unchanged.